When casting strip according to the twin roll casting process, the oppositely-rotating casting rolls of the caster delimit the longitudinal sides of the casting gap. On its narrow sides, however, the casting gap is sealed by a side plate in each case.
The side plates are usually comprised of an insert and a support plate, which bears the insert. The insert in this case is usually made from refractory material and is formed so that it partly covers the front sides, associated with it, of the casting rolls and completely covers the narrow side, to be delimited by it, of the casting gap.
The relative movements, necessary in the casting process, between the side plates and the front sides of the casting rolls as well as the contact with the molten metal moving through the casting gap inevitably lead to wear of the side plate due to abrasive wear. This is particularly true if the contact between the side plates and the molten metal or the casting rolls is realised by means of an insert made from refractory material.
In order that, in the case of side plates equipped with such inserts, it is ensured that the side plates perfectly fulfil their sealing function also at the start of the casting process, it is necessary to grind in the inserts before the casting process begins. For this purpose the respective side plate is adjusted while the casting rolls are rotating until the inserts of the respective side plate lie with the required contact pressure against the respective associated front sides of the casting rolls. Thereupon abrasive wear of the insert material occurs in the region of the contact surfaces.
This condition is maintained until the casting rolls are ground in to a certain required depth into the refractory material of the insert and the insert lies closely fitting against the front sides of the casting roll. The inserts of the side plates now have one section, the so-called “positive insert”, projecting into the casting gap, and two sections, the so-called “insert grinding surfaces”, adjacent thereto, springing back in relation to the positive insert, with which sections they lie against the front sides of the casting rolls. Here the positive insert with its lateral edge surface covers a narrow margin, adjacent to the respective casting roll front side, of the peripheral surface of the casting rolls in each case.
Because the side plates, possibly by means of their respective insert, are in direct frictional contact with the cooled casting rolls during the casting process, heat is constantly removed from them. Therefore, during the casting process, the temperature of the side plates is usually lower than the temperature of the molten mass coming into contact with them. Consequently heat is also removed from the molten mass when it touches the side plates. The heat loss in this case can be so great that molten mass solidifies on the respective positive insert. In the case of side plates equipped with an insert made from refractory material, such “parasitic solidifications” can take place, particularly in the region of the positive insert.
Parasitic solidifications can cause strip defects, such as thickness discontinuities (deviation from defined tolerances), insufficient through-solidification (bulging aspects) and material tears (strip edges). In extreme cases they may require the casting process to be aborted.
It is known that wear of the insert due to contact with solidifying molten mass can be reduced by moving the insert during the casting process in a precisely pre-determined way relative to the molten mass that is to be cast. For this purpose the side plates are oscillated rotationally about a centrally arranged axis. With this rotational oscillation it is equally possible to counteract solidifications in the lower third of the insert and to counteract the wear increasing downwards. Examples of this procedure are described in Japanese Patents JP 03-174954, JP 05-237603 or U.S. Pat. No. 5,188,166.
In order to ensure sound, integral contact of the side plates on the front sides of the casting roll associated with them in each case, the side plates are usually moved with regulated force against the front sides of the casting roll before the start of the casting operation. If a certain force is reached, the casting process is started. In the proceeding casting operation, the contact pressure necessary for sound sealing is then maintained.
Due to different thermal dilatations, unevenness of the surfaces of the side plates coming into contact with the front sides of the casting rolls, axial misalignment of the casting rolls and other reasons, in practice it is necessary during the casting operation to constantly regulate and readjust the contact pressure, under which the side plates are kept pressed against the front sides of the casting rolls. This is valid in particular for such side plates, wherein sealing takes place by means of an insert, coming into contact with the casting rolls, made from refractory material. In particular constant regulation of the contact pressure is necessary due to the wear of the inserts, which increases as the casting progresses.
The necessary contact pressures are usually applied by actuators, which deliver a force aligned toward the casting gap. Their effective direction is usually aligned substantially axially parallel to the axes of rotation of the casting rolls.
A fundamental problem when regulating the positioning of the side plates against the casting rolls is that, while the casting rolls are rotating, vertical forces develop in consequence of friction between the side plates and the front sides of the casting rolls. The latter is valid in particular if the side plates have an insert made from refractory material, lying against the casting rolls.
The vertical forces increasing proportionally to the axial force applied to the side plates or proportional to the amount of friction have a strong influence on the pressure distribution along the contact arc surfaces, by means of which the side plates lie against the front sides, each associated with them, of the casting rolls. With increasing vertical force the axial pressure rises in the upper region and falls correspondingly in the lower section of the side plates, the force detected on an axial cylinder remaining unchanged. The result of this is that the side plate wears more heavily in its upper section than in its lower section.
The same is also true in the horizontal direction if different friction conditions prevail between the respective front sides of the casting rolls and the side plates. In the case of side plates equipped with an insert made from refractory material and coming into contact with the casting rolls, in practice a difference in thickness of several millimetres between the upper and lower section, or left and right section relative to the vertical, of the insert arises due to the wear that accompanies this. Consequently the total insert thickness cannot be used but the life span of the inserts is determined by the section in which the maximum wear takes place. Furthermore the wear, which is particularly heavy in the region of certain sections, means that as the casting progresses, the positive insert projects further and further into the casting space, as a result of which the dimensional precision of the cast strip is impaired in its width direction, in particular when strip is cast over a long period.
The problems described above arise particularly intensively in the case of such twin roll casters, wherein for reasons of space and cost savings the axially adjustment of the side plate aligned against the front sides of the casting rolls is carried out by only one axial cylinder. Casters equipped in this way are described for example in U.S. Pat. Nos. 5,588,479 and 6,296,046.
In International Patent WO 05/023458 it is proposed to counter these disadvantages by distributing the axial contact pressures in a way that is adapted to the respective wear pattern arising on the respective side plate during casting. The technical cost necessary for this, however, is substantial, since not only complicated control and regulation techniques but also actuators which work in a sophisticated manner must be made available.